What Is A Cult?
The word 'cult' usually refers to a small group of people with radically different ideas. Certainly some cults can be very dangerous especially when they carry weapons to defend themselves. Jehovah's Witnesses do not do this. They do stand up for what they believe is right and this is very commendable. However, as a group they do not take up arms against people who threaten their way of life. They believe that their God Jehovah will protect and defend them from their enemies. See Enemies of Jehovah
There are many positive reasons why people are attracted to the Jehovah's Witnesses. If it was all bad, no one would join and no one would stay. Most Jehovah's Witnesses do not see anything negative about their faith. They are proud to be different and there's a sense that even though others may look down upon them, they are best religion in the world. The only time a Jehovah's Witness gets to see the negative aspects of their religion, is when they decide to leave.
Generally speaking, people think of cults when they see a group like this.
- Everyone believes in the same things
- People are afraid to question beliefs or disagree
- Beliefs are radically different than those of people outside their group
- Everyone has complete trust in their leader or leaders
- Control is tight. Rules are strict and must be followed
- Cult members feel superior to outsiders
- It is difficult to leave 
Why Do People Say Jehovah's Witnesses Are A Cult?
The word cult is merely a label. Usually people think of cults in a negative way. However in modern times, the word has been used loosely to refer to any group that is dedicated and on a mission to promote beliefs that are not mainstream.
Are Jehovah's Witnesses a cult? Some say they are for these reasons. Jehovah's Witnesses pride themselves in being 'like-minded' and 'speaking in agreement'. See Mind Control They feel their understanding of the Bible is superior to people who belong to other religions. See the Watchtower Sept 1, 1989 Jehovah's Witnesses refer to themselves as being 'in the truth'. Anyone who is not a JW, is said to be 'out of the truth'. Unity of beliefs is a requirement if you want to be part of their organization. 'Politicking' or disagreeing with the Governing Body of Jehovah's Witnesses is not allowed. There are negative consequences for questioning the authority of the Governing Body or breaking the rules. And it's very difficult to leave because of their doctrine of disfellowshipping and shunning. 
Are The Jehovah's Witnesses a Dangerous Cult?
For the most part, as a group, Jehovah's Witnesses are peaceful people. They don't go to war. They refuse to hold any job that requires them to carry a gun. And they are very conscientious about treating non believers with respect. So in that sense, no, most people do not consider Jehovah's Witnesses to be dangerous. Generally speaking they are harmless to outsiders. However there are always exceptions to the rule.
The question of being dangerous is like saying, are Catholics or Baptists dangerous? Of course they are not. However, individuals can stray from the teachings of their religion and cause some damage if they choose to do so. What an individual does, goes beyond the control of any church.
Why Do People Say Jehovah's Witnesses Are A Dangerous Cult?
Critics believe that the dangerous part of the Jehovah's Witnesses is being one. Why? Because according to some researchers, the dynamics of their group can lead to greater cases of mental illness. Also the group has a very strict code of conduct. See 141 Things Jehovah's Witnesses Can't Do. Restrictions upon personal freedoms can lead to unhappiness, depression, guilt, and a feeling of hopelessness. In that sense, the beliefs of Jehovah's Witnesses can be dangerous for certain individuals who are independent thinkers.
Some also point to the fact that Jehovah's Witnesses refuse blood transfusions. Critics say that they have misinterpreted the Bible and in doing so they are putting their lives at risk. Jehovah's Witnesses disagree. They strongly believe that blood transfusions are wrong even though this belief has absolutely no support in the Bible. See Blood Transfusions
People who choose to join this religion as adults, willingly have given their personal freedoms away to serve God by being faithful to their religious leaders. That's everyone's right. However children born into this faith don't have a choice. If you're born into the Jehovah's Witness faith, and get baptized at a young age, leaving the organization as an adult is not easy. The consequences of leaving not only affects young adults, but also parents, brothers, sisters, uncles, aunts, and other loved ones within their church. In other words, the practice of disfellowshipping people who leave, punishes everyone, - including faithful Jehovah's Witnesses who choose to stay in the organization. 
As long as a Jehovah's Witness follows the rules, they're accepted and loved. However any disagreement within the organization has exactly the opposite effect. See disfellowshipping and shunning. The practice of excommunicating individuals from their religion, can destroy families and life long relationships. 
People who leave the organization are called 'apostates'. Even though non believers are treated with love and respect, apostates are shunned and rejected. See Apostate Jehovah's Witnesses 
If you're interested in joining the Jehovah's Witnesses, please take a look at the negative and positive aspects of belonging to this controversial faith. Before you make a decision, please consider this eye opening article, Should I Get Baptized As A Jehovah's Witness?
1. Compare how 8 JW's answer the question about whether they are a cult. Then watch this video in which the organization tells JW's they must shun their parents, children, and other relatives who leave. The Watchtower has been teaching this since 1952 and they've done a pretty good job keeping the truth hidden from the general public.
A Better Way.